Sri Lankan fishermen have rescued four Somali men who were drifting in a small boat off the island's coast -- where pirates from the African nation are known to be active -- police said on Tuesday.

A Sri Lankan trawler found the men last Wednesday in international waters in the Indian Ocean, thousands of kilometres from Somalia, a police official said.

"The (Somali) men were handed over to us today," police spokesman Ajith Rohana said. "What we gathered from the fishermen is that the Somalis were not armed at the time of the rescue. But, we can't say if they are pirates or not."

In January last year, Somali pirates operating in the Indian Ocean region killed two Sri Lankan fishermen and seized three men along with their trawler.

A Bolivian-flagged vessel seized by Somali pirates in Maldivian waters two months ago was later freed with its Iran-bound sugar cargo.

Two decades of lawlessness have carved up Somalia into mini-fiefdoms ruled by gunmen and militia, encouraging rampant piracy.

At least 40 vessels and more than 400 hostages were still being held in or just off Somalia at the end of last year, according to the Ecoterra International group which monitors piracy in the region.